The present invention relates generally to computer measurement systems. More specifically, the present invention relates to the measurement of features on photographic masks used in semiconductor manufacturing.
The recent introduction of advanced sub-micron sized semiconductor devices require reduced critical dimensions and increased packing densities. At these sub-micron sizes and high densities, even defects and imperfections as small as 1 micron and below are problematic and need to be detected and evaluated. Imperfections in the reticle generated by a photographic mask manufacturing process are one source of defects. Errors generated by such a photomask manufacturing process have become an important issue in the manufacture of semiconductor devices at these sub-micron sizes. Defect inspection techniques for masks are therefore becoming to play a more important role in mask making and quality assurance.
Thus, it is becoming increasingly important to be able to identify and to correctly size mask defects, line widths, heights of edge defects and other features that are under 1 micron in size. Accurate sizing of these features allows masks that are below specification to be repaired, and prevents the needless and costly hold up of masks that do meet specification. However, one of the problems of assessing reticle quality at these sub-micron levels on an automatic inspection system is that the size of these features cannot always be accurately, quickly and cost-effectively measured in a production environment.
Although mask makers typically repair most defects found at early inspection stages, invariably, defects are found at later inspection stages (such as after pelliclization of the mask has occurred). These late stage defects are sized and classified relative to a defect size specification, the size at which device performance is deemed to be affected.
Currently, defects found by automatic inspection tools are classified in one of the following categories by a human operator: (1) a real defect is a hard or soft defect that exceeds the defect size specification, (2) a sub-specification defect is a random or process-related defect below specification that is within a safety margin, and (3) a false defect is a defect detected by the inspection tool with no apparent cause.
Classification of the above types of defects is largely a subjective activity based upon operator skill. However, as defect size specifications diminish, the distinction between real and sub-specification defect classification has become increasingly difficult. For example, as the line width on sub-micron masks approaches 0.1 micron, the ability to measure defect sizes at 1 micron and below becomes very important. Current production machines have an accuracy of 0.1 micron to 0.2 micron, but this is not sufficient.
It has long been known that mask inspection tools are not measurement tools and that the size information provided by these tools has limited value for measurement-based defect classification. Consequently, many mask makers have incorporated measurement aids at the inspection station or have moved the mask to a more suitable measurement tool in order to make classification decisions. Measurement aids used at the inspection station include calipers, grids, and software based video image markers such as gates, scales, grids, boxes and circles. These aids are fairly rapid, but ultimately require the operator to xe2x80x9ceyeballxe2x80x9d the boundaries of the defect. This activity is very subjective and can lead to an error in the measurement of the defect.
For example, particle size is conventionally measured by measuring the distance between opposite edges of the particle. Once a defect is identified by an inspection machine, the operator uses a video microscope and a television camera to position a cursor on one side of the defect and another cursor on the other side of the defect. The operator must judge for himself the exact boundaries of the defect and must place the cursors where he sees fit. At this point, the operator pushes a button and the software blindly computes the distance between the two cursors in order to supply a rough approximation of the diameter of the defect. This technique has many disadvantages.
Firstly, this measurement technique is operator dependent in that the operator must manually position the cursors on the boundaries of what the operator believes to be the defect. The operator may misjudge the type of a defect, its boundaries, or may simply misplace a cursor even if the defect is visible. The software then blindly calculates the distance between the cursors, without regard for the type of defect, its true boundaries, etc. The above technique may be performed with a standard video microscope and has an accuracy of about 0.1 micron, but is completely subject to the operator""s skill level and interpretation. This technique is also unable to calculate an area for a defect.
Another difficulty with light measurements of features less than 1 micron in size is that the wavelength of photons begins to interfere with the measurement of these smaller and smaller feature sizes. Current techniques do not adequately address the nonlinearities associated with such measurements.
Alternatively, the mask may be removed from the automatic inspection tool and relocated on a more precise and repeatable measurement tool. However, this approach involves removing the mask from production, relocating the defect, and is thus impractical in a production environment. This technique is also costly, time-consuming and increases the handling risk. For example, an atomic force microscope (AFM) may be used to measure defect sizes; such a microscope is extremely accurate but is very slow, very expensive and is still subject to operator interpretation.
One approach that has been taken that uses calibration of an automatic inspection system in order to size defects is described in xe2x80x9cCharacterization Of Defect Sizing On An Automatic Inspection Stationxe2x80x9d, D. Stocker, B. Martin and J. Browne, Photomask Technology and Management (1993). One disadvantage with the approach taken in this paper is that it only provides a technique for measurement of defects of 1.5 microns and greater. Such sizes of defects would produce a linear relationship between reference sizes and actual measured sizes, and the paper does not account for defects less than 1 micron that would produce a non-linear relationship. Also, the technique does not allow for individual calibration data for particular types of defects.
Therefore, an objective feature measurement tool is desirable for use with a photomask inspection tool that can provide reliable and repeatable measurements of defects and other features of less than about one to two times the microscope resolution (or about less than one micron for optical microscopes). It would be especially desirably for such a tool to operate in a fast and highly practical manner in a production environment.
Furthermore, at subresolution sizes it can be extremely desirable to determine the opacity of a feature or defect. For example, measurement of the area or other dimension of a feature will be affected if the feature is not 100% opaque. That is, features that are less than opaque make flux-based measurements of area, diameter, height and/or other dimensions more difficult. For example, a light measurement of a half-transparent spot defect produces more flux from the defect than a perfectly opaque defect; thus, a measured area would appear to be about one-half of the true area. In other words, a less than 100% opaque feature would appear smaller than it truly is. It would be desirable to determine the opacity of the feature or defect to not only correct for a measured dimension of the feature, but also to help determine what the feature is composed of. It would be especially useful to determine the opacity of features having a size that are less than about twice the wavelength being used.
As previously explained, it is desirable to be able to measure the area, and hence the diameter of a feature or defect that is extremely small. Measurement of diameter in this fashion is especially useful when the feature is round, or when it can be assumed that the feature is round. However, features and defects are not necessarily always round; for example, features of about less than 1 micron in size may appear round when in fact they are not due to the blurring effect. In a variety of situations, it can be important to know more specific dimensions of a feature if the feature is not round. For example, given two aluminum lines on a silicon substrate, an edge defect in one line might produce a short across to the other line. However, measurement of the area of this edge defect (to produce an estimated diameter) will not necessarily indicate that the edge defect is in danger of shorting out the two lines. This is because a rather elongated edge defect that is in no danger of shorting the two lines will have the same area as a narrower, taller defect that may in fact produce a short. Determination of the diameter of a defect by measurement of the area of the defect may not always indicate the true height or width of a defect, especially if the defect is not round.
Traditionally, a deconvolution technique using a Fourier transform to a frequency space has been used to measure height and width of features that are less than 1 micron in size. However, this technique has typically not produced accurate results for features of this size. Therefore, in order to more accurately classify features and defects it would be useful to be able calculate the height and width of a feature or defect with a high accuracy. Determination of height and width of features having sizes that are less than about twice the wavelength being used would be especially useful.
It would also be desirable to be able to determine the radius of curvature of a corner of a line, especially at sizes that approach, or are less than, the wavelength of light or the particle beam being used where blurring is a problem.
The present invention discloses a measurement tool that provides an objective, practical and fast method for accurate sizing of mask features found with an automatic inspection tool (such as a video inspection machine). Diameters of defects and dimensions of other features can be measured by using gray scale image information provided by the automatic inspection tool. The present invention may be used while the photomask is in-place at the inspection station, and there is no need for the mask to be removed to a different machine for measurement. Also, an operator is able to quickly outline a general region around a feature to be measured without the need for the operator to judge the size of the feature. The dimension of the feature is then automatically identified and measured quickly by the measurement tool of the present invention.
Benefits include avoiding repairing masks within specification, and equivalent results whether measured by customer or supplier (when calibrated with the same reference). Also, marginal defects are accurately measured and classified as fatal or not, and sub-specification defects are stored and documented with printouts including picture and text. Operator productivity and tool utilization is improved by rapid measurements taking place at the inspection station. Repair quality may also be checked by taking a line width measurement.
The disclosed measurement tool objectively and repeatedly measures defects (spots, holes, intrusions, extensions, etc.), line widths at any angle (transmissive and opaque vertical and horizontal lines), heights of edge defects and dimensions of a variety of other features for determining photomask disposition. The measurement tool operates automatically and is not dependent upon operator judgement. Defects from 0.1 to 1.5 microns can be measured, repeatable to 0.02 microns and accurate to 0.05 microns with a typical AFM calibration. Line widths less than 1 micron can be measured, repeatable to 0.05 microns and accurate to 0.1 micron with a typical AFM calibration. Measurements from an AFM, Vickers or OSI machine may be simulated by calibrating to their standards. Additionally, the measurement tool provides automatic measurements in 1 to 5 seconds (including operator actions).
By analyzing a VERIMASK image and correlating measured pixel widths of the feature to the known size of the defect or line width in microns, the inspection machine, its optics and electronics, video hardware, and computer hardware and software may all be calibrated. There is no need to calculate variances due to individual camera response, ambient light, individual machine characteristics, or other variables. Thus, a measurement of an actual defect (for example) using the same hardware to return a diameter in pixels may be directly compared to a particular calibration curve for that type of defect in the calibration database in order to return an extremely accurate width of the defect in microns.
In one embodiment, test features of known sizes (measured from a VERIMASK, for example, a plate with programmed features and standard sizes for each feature, made by DuPont Photomasks Inc., of Round Rock, Tex.) are first measured in order to serve as a calibration gauge for the measurement tool. Once dimensions in pixels for these features have been determined, these measured dimensions for various sizes of features are plotted against the true sizes in microns of the features (known beforehand). This calibration procedure yields a polynomial curve that is used for future measurements of features of unknown sizes on the same hardware, in order to calibrate the measurements. In a further embodiment, a separate calibration curve is developed for each type of defect and for each variety of line width. Separate calibration curves for each feature correct for optical characteristics of the system in relation to a particular feature and likely skews in the reference measurements due to individual features.
In another embodiment of the invention, a non-linear calibration polynomial curve is produced relating measured sizes of features from a particular machine to known reference sizes of these features. A light measurement is performed on features of unknown sizes on the same machine to return a measurement in pixels. This value in pixels may then be compared to the above described calibration curve in order to return an accurate value of the size of the feature in microns. In a further embodiment, calibration curves are developed for each type of feature, and light-measured features of a particular type are compared to their specific calibration curve. Advantageously, a calibration curve yields a more accurate measurement.
In another embodiment of the invention, the type of a feature is determined by first forming a bounding box around the feature. Analysis of whether the box touches a user region of interest yields either an isolated defect or other feature. Analysis of light transitions for other features yields a determination of the types of these other features. The feature type is determined automatically.
In yet another embodiment, a good quality source image of a feature is found by subtracting a reference image. A light intensity distribution profile is first developed for a region surrounding the feature. If the profile is not of good quality, then a reference image of the feature is obtained and subtracted from the feature image in order to produce a good quality source image.
In another embodiment of the invention, multiple regions of interest are formed surrounding a feature and an intensity profile is developed for each region of interest. A total light flux measurement is calculated for each profile, and one of the light flux measurements is chosen as the best flux value. A good quality profile is chosen such that the total flux measured from the profile is proportional to the area of the defect. Multiple regions allow for angled lines.
In a further embodiment of the invention, a region of interest surrounds the feature and a profile for the feature is produced by summing columns of pixels across the feature site in the region of interest. A baseline intensity value is determined for the profile and is subtracted from the profile in order to determine the total flux passing through the feature. Subtraction of a baseline removes background intensities and obviates the need to obtain a reference image.
In yet another embodiment, the height of edge defects may be measured accurately. The height of a defect is important to measure especially if the defect occurs on an edge, such as a bump on a line or a bite into a line. The height refers to the two-dimensional distance that an extension sticks out from a line, or how far an intrusion bites into a line (as opposed to the three-dimensional thickness of a defect). The measurement of height provides an indication of how close the defect extends to adjacent lines in two dimensions. One column of light intensities one pixel wide in the profile is summed to determine an area of the feature that corresponds to the height of the feature.
In a further embodiment of the invention for calculating line width, a region of interest surrounds a line portion and a profile for the line portion is produced by summing columns of pixels across the line portion in the region of interest. The total flux is then determined for the profile and is used to find the area of the line portion,. The area of this line portion divided by a dimension of the region of interest yields the line width.
In another embodiment, calibration data is developed for a line in order to calculate a line width. A light measurement is taken of a line width of known size that is at a distance of less than one resolution unit from another line. A full-width half-maximum technique is used to assist in calculating the line width. The measurements are repeated for number of line widths of different sizes in order to develop calibration data that relates light-measured values for line widths to the known widths of the lines. The calibration data may be represented as a non-linear polynomial curve that can be referenced by future measurements of line widths of unknown sizes that are less than one resolution unit from other lines in order to return more accurate results.
In one other embodiment, non-linear calibration data is developed for line widths of lines that are less than one resolution unit from other lines. An actual measurement of a line width of unknown size that lies less than one resolution unit from another line may reference the calibration data in order to return a more accurate result. The use of calibration data developed using a full-width half-maximum technique is advantageous for lines less than one resolution unit apart due to non-linearities associated with measurements at these small dimensions.
In an additional embodiment, an image of a feature is extracted from the video output of an automatic inspection tool. The operator draws a very rough box around the general feature site without needing to gauge the size of the defect. Pin point accuracy is not needed. The measurement tool automatically picks out the feature from within the region of interest identified by the operator, identifies the feature, and calculate the area, width and height of the feature. These measurements are completely independent of the size of the region of interest identified by the operator, thus removing operator judgement from the measurement.
Thus, by providing an extremely accurate measurement of mask features, the disclosed measurement tool helps to avoid unnecessary mask repairs and allows for improved process control. Also, operator variability is eliminated, and overall productivity and mask throughput is increased due to the accurate measurements in-place and documentation produced in seconds. Because the measurements are automatic, operator training is minimal. Repair quality can also be quickly checked by using line width measurements.
In another specific embodiment, the present invention determines the opacity of a feature or defect to use in correcting area or height measurements as well as helping to determine what the feature is made of. While prior art techniques may produce opacity values for relatively large particles, the present invention is especially useful for dimension measurements of less than perfectly opaque features that are under 1 micron in size.
The present invention is also able to measure specific dimensions (such as height and width) of features that are under about one to two times the microscope resolution. Features and defects such as spots, holes, line widths, edge defects, etc., can all be accurately sized at this subresolution size even though blurring is present. The prior art has had difficulties in measuring the width of a feature at subresolution sizes. In a specific embodiment of the invention, the curvature of the intensity profile of the feature is used in conjunction with a curvature-width lookup table in order to produce the width of the feature. Advantageously, the present invention develops curvature-width lookup data by determining the area and then the diameter (or width) of known round defects. The curvature of the profile of the round defect is also determined. The width and curvature are then entered as a data point in the lookup table, and the process is repeated for defects of other sizes to fill out the table. Alternatively, width of a feature could be directly measured on another machine for use in the lookup table.
In another embodiment of the invent ion, the height of a feature is accurately determined for features having sizes less than subresolution. In one embodiment, a width-height multiplier is used to correct the height due to the blurring associated with sizes close to image resolution. An opacity correction is also used to correct the height of features that are less than 100% opaque. Width-height multiplier data is developed by measuring the area of a known round defect in order to get its diameter (or height). The height of the same defect is then measured using the techniques described herein (techniques that are applicable to defects of any shape). The height determined from the area (which is assumed to be more accurate for a round defect) is then compared to the measured height to produce a height multiplier. This process is repeated for round defects of a variety of sizes to produce the width-height multiplier data. During an actual measurement of the height of a feature of unknown dimensions, both the height and width are measured using the techniques described herein. Then the width-height multiplier data is referenced using the measured width to produce a height multiplier for that width. The height multiplier is multiplied with the measured height to produce a corrected height.
Preferably, profiles are taken parallel to lines for edge defects and nearby isolated defects. Thus, a xe2x80x9cwidthxe2x80x9d value is obtained using the curvature technique described above, and a xe2x80x9cheightxe2x80x9d value is obtained using the height multiplier technique from above. For isolated spot defects that are about at least two blur distances from a line, a profile could be taken in any orientation and thus a xe2x80x9cwidthxe2x80x9d measurement could be used to determine both dimensions by forming profiles orthogonal to one another. Similarly, a xe2x80x9cheightxe2x80x9d measurement could also be used to determine both dimensions for such isolated defects.
It should be appreciated that the dimensions of height and width are arbitrarily imposed on the orientation of a two-dimensional feature, and the two terms can be interchanged without effecting the operation of the present invention. In a preferred embodiment, width refers to a dimension parallel to a nearby line, while height refers to the dimension orthogonal to width.
Frequent reference is made herein to the applicability of the invention for sizes of features less than about 1 micron; this range applies to visible light, where the wavelength is about 0.5 micron. For other wavelengths, the present invention is generally suitable for features having sizes less than about two times the blur distance of the optics being used, or for features that are less than about twice the wavelength being used. The invention is especially suited for features having sizes close to or less than the wavelength being used.
An embodiment of the present invention is able to accurately determine the radius of curvature of corners of features on a variety of media, and especially at subresolution sizes.